Method for making molds for inlays and crowns



March 17, 1931. FgsMEvER 1,796,470

IETHOD FOR MAKING MOLDS FOR INLAYS AND CROWNS Filed Dec 12, 1 .927

\ I l l 2 9' 6 I441: enlo'r E v J1 QZ W ,j wflwr 5 Patente d Mar. 17, 1931 i I UNITED STATES PATENT OFFICE FREDERICK S. MEYER, OF'MINNEAPOLIS, MINNESOTA METHOD FOR MAKING MOLDS FOR INLAYS AND CROWN S Application filed. December 12, 1927. Serial No. 289,427.

This invention relates to an improved easily disintegrated and scorched by the ap 'method for making cast metal dental inlays lication of excessive heat. In methods and crowns; iieretofore practiced for making dental fill- While in the last twent years considerable ings it has een impossible to use an investprogress has been 'made 1n the formation of ment compound containing in excess of dental fillings, the methods carried out herethirty three and 0ne-third per cent plaster tofore have not been entirely, satisfactory with any degree of proficiency because, and for several reasons. In the first place, it has only because the steps of said former methods heretofore been impossible to produce a where the wax and moisture were eliminated 1 crown or other filling which has binding from the mold had to be carried out at too points, where the casting could be taken dihigh a temperature. rectly from the mold and fitted perfectly to There has been in the last twenty years the tooth or to a porcelain or steel model. only two widely known methods for elimi- The margins of crowns and inlays produced nating the wax and moisture from the mold.

13 by methods heretofore known do not fit per- In the first of said methods, namely the Tag- I fectly and could only be made to approxi gart method, sometimes called the carbonimately fit by careful grinding. zationmethod, an. extremely low plaster con- In the case of crowns or other fillings of tent investment was used and the mold was the type having points that tend to bind, a subjected to heat sufficient to melt the wax dentist would always have to swedge the inand allow it to be absorbed into the invest- 7 lay on a metal die or pound the inlay into ment material and by continuous heat at place compressing the dentinal cells of the high temperature to be evaporated from the tooth and thereby endangering the life of mold. In this method use of investment mathe tooth or increasing the danger of fracterial containing over twenty per cent plas- 975 ture. ter content was extremely hazardous. The partial failure of methods heretofore In the other well known method for formused was due to several causes. In the first ing fillings, namely the Knapp method,theinplace, none of the earlier methods made proventor desired to use a higher plaster content vision for compensating. for shrinkage of and did use a plaster content in his investgold in the process of casting. Careful tests ment material up to'thirty three and onehave shown that gold shrinks when congealthird per cent, which produced harder molds ing from molten to solid state approximately than had been produced before his discovery. one and one-half per cent. It will be obvi- In the Knapp method the wax was forced out ous that in crowns and inlays adapted to surfrom the mold by steam produced by the round portions of the tooth this shrinkage moisture in the mold itself. Flame or heat is sufficient to cause the inlay to bind and was directly applied to the mold and where thereby fail to be properly seated. The the wax model was large it was often imposmargins of the fillings were imperfect since sible to eliminate all the wax without using the shrinkage of the gold prevented the gold excessive heat. A dentist was required to casting from filling out over and fiush'with work within narrow limits and even then the the tooth as did the wax model. investment was often injured due to scorch- Another cause for failure in methods hereing or disintegration of the plaster because of tofore carried out was due to the fact that the excessive heat used. The plaster content investment material ofvsufiicient rigidity and of investment material which could be.utistrength to resist the binding action of the lizd in this method was necessarily limited congealing gold to any appreciable extent and was not sufiicient to produce a mold could not be used with any degree of-safety. which would adequately resist the binding It is a well known fact that plaster of Paris action of the congealing gold to produce desets to form a very hard and smooth surface. sired results. By binding action of co'ngeal- It is also well known that plaster of Paris is ing gold I mean the action caused by the shrinkage of the casting between two binding points.

It is the main object of this invention to provide a new method of comparative simplicity for obviating many of the causes of failure in the old methodspracticed and for producing very nearly perfect crowns and inlays.

It is a more specific object to provide a novel method for preparing dental fillings, such as crowns and inlays in which provi sion is made to adequately compensate for the shrinkage of the congealing gold.

A further object is to provide a novel process of the type above described in which the elimination of wax and moisture from the mold may be successfully carried out without endangering the mold or the ingredients of the investment material.

Still another object is to provide a method for forming dental fillings-wherein compensation will be made for the shrinkage of the gold or other casting metal and wherein investment material affording the hardest and smoothest type of mold may be utilized without danger of impairment due to removal of the \Vax model or moisture therefrom.

Another object of this invention is to provide a convenient and efiieient apparatus for carrying out the steps of eliminating the wax and moisture from the mold without injuring the investment material.

These and other objects and advantages of the invention will be apparent from the following description made in connection with the accompanying drawings in which like reference characters refer to similar parts throughout the several views, and in which,

Fig. 1 illustrates a wax model for an inlay mounted on a sprue or crucible former, showing the first step in the investment of the model;

Fig. 2 is a vertical cross section showing the inner investment applied to the wax model and illustrating the step of applying the outer investment Fig. 3 is a view partly in vertical section, illustrating a form of oven or so-called wax expander which is maintained thermostatically at a predetermined temperature;

Fig. 4 is a view taken partly in side elevation and partly in vertical section illustrating a convenient and preferred form of apparatus for carrying out the elimination of wax and the elimination of moisture from the mold;

Fig. 5 is a fragmentary bottom plan View illustrating a suitable thermostatic control for maintaining my eliminating heater at a predetermined temperature; and

Fig. 6 is a wiring diagram of said heater.

What I accomplish in a general way by my improved method in preparing east inlays is to expand the-wax model as it is being inmeaevo vested and during the time the investment is setting to compensate to a large extent for the shrinkage of gold when the casting is congealing; to further use inner investment which will set very hard and will have a minimum degree of compressibility and which moreover will form smooth casting surfaces; and lastly to carry out the elimination of wax from the mold in a new and improved manner without the direct application of flame or heat to the sides of the mold and to eliminate the moisture from the mold at a lower temperature whereby the dentist will not have to work within narrow limits as to time in effecting said elimination but may work within relatively wide limits and still produce successful results even though a dense hard investment is used.

In carrying out my process the usual wax model is first prepared and I prefer to chill the wax model as much as possible while held in position in the cavity. I useinner and outer investments but before mixing my inner investment I mix my outer investment, using water which has been heated to a temperature between 100 and 140 F., depending on the nature of the casting to be made. The higher temperatures within said range are used for the more bulky castings. For this outer investment any suitable material may be used such as well known commercial investments containing silex, graphite and a relatively small proportion of plaster of Paris usually, between 12 and 18% provided it is of such na ture that it requires a comparatively long time for the initial setting. The outer investment material should be of such nature that it can be oured satisfactorily any time for a period of our or five minutes after it has been mixed. The said outer investment is mixed to a heavy cream consistency and placed in a receptacle, usually a rubber bowl 8, which is subjected to heat from below at a constant temperature of approximately 140 F. I find it convenient to provide an oven which I shall hereafter refer to as my wax expander for carrying out the step of my process just mentioned and for also carrying out other steps to be later mentioned. My wax expander is illustrated in Fig. 3 of the drawings and is in the nature of an oven provided with shelves 7 and having a recess in the top in which the receptacle 8. containing the outer investment is disposed. My wax expander is thermostatically controlled and maintained at a predetermined temperature and I find that very successful results may be had if the temperature is kept at approximately 140 F. As shown in Fig 3, my Wax expander is also provided with an aperture 9 in its top through which a suitable bottle or other receptacle 10 may be inserted for containing the water to be utilized for my outer investment. A cap 11 may cover aperture 9. The investment rings R to be later utilized are also preferably kept within tle wax expander for maintenance at the temperature.

I next place the wax model upon a sprue and the sprue s set up in the usual crucible former. My lnner investment is then mixed on a glass slab, and this may be prepared by adding a quantity of cold water to the investment powder. Heated water is not used since the wax model would be softened and distorted by the weight of the mix. In my method it is of great importance to use an inner investment which will be relatively hard, dense and present a smooth casting surface. After extensive experiments, I find that an inner in vestment comprising silex and a relatively large proportion of plaster of Paris produces the most desirable results. The nature of my wax and moisture elimination steps is such as will hereafter more fully appear that I am able to use an inner investment containing desired between 43 and ninety per cent of plaster of Paris, which would be totally unsuitable for any methods carried out up to this time. Excellent results have been obtained by the use of investment containingfrom sixty six and two-thirds per cent plaster of Paris and 33%% silex. This heavy mixture is preferably first applied to the cavity side of the wax model by a small brush as shown in Fig. 1, and the crucible former is then vibrated by rasping with a file. This rasping causes a very dense mix next to the wax model as Well as eliminates the bubbles. The cavity portion is again filled up and the rasping step repeated. A little of the mixture is painted over the remaining surface of the wax model and the whole model is then completely covered up with the dense investment down to the crucible former, this dense inner investment being from one to three sixteenths of an inch thick.

After the wax model has receivedits inner investment I immediately remove one of the' investment rings from the wax expander which has attained the temperature of 14:0 F, place the ring on the crucible former and join therewith and pour the relatively thin outer investment into the investment ring from receptacle 8, filling the ring in the usual time this hardening step and under the conditions above set forth, find that from thirty to fifty minutes is approximately the time necessary to properly harden tllQ'IIlOld for casting. After exhaustive experiments I have found that within the range of temperatures above noted and when invested in the manner described, the wax model will be properly expanded to compensate to a large extent for the shrinkage of the congealing gold.

After hardening the mold is taken from the wax expander and the crucible former and sprue are removed and the closed end of the mold is leveled 0a. The closed end of the'mold is next subjected to the action of confined water with the sprue passage disposed outwardly and heat is applied to the water container to cause the water to pass through the mold under pressure. It will be noticed that no direct application of heat takes place on the sides of the investment ring or mold. The application of heat to the water container causes small amounts of steam to be generated which force the hot water through the mold softening and disintegratin the wax model and washing the wax in softened state through the mold and out therefrom through the sprue hole.

A convenient and efiicient apparatus for carrying out the wax eliminating step in my process as well as for effecting the next step to be described is illustrated in Fig. 4'. This apparatus includes an electric eliminating heater comprising a substantially cylindrical outer casing 12 provided at its upper end with a truncated conical top 12a having a relatively large concentric aperture through the upper end thereof, casing 12 being fur- M ther provided with a horizontal partition 12?) disposed some distance above its base.

A cup-shaped oven 13 is disposed within heater casing 12 with its open end disposed upwardly and fitting within the open end of the truncated conical top 12a. Oven 13 may be supported in any suitable manner, such as by the bolt 13a projecting upwardly from partition 12?) and having threaded engagement therewith. A suitable-heating element E illustrated as of spiral form may be wound about or otherwise applied to the outer surface of oven 13, a relatively thin sheet of insulation material preferably being interposed between heating element E and said surface. Suitable connections are provided for setting up a flow of electricity through element E and a switch S may be mounted on a convenient portion of heater casing 12 to control the flow of current.

The bottom portion of heater casing 12 below partition 1% constitutes a thermostatic chamber the temerature of which varies proportionately with the temperature of the heating chamber disposed immedlately thereabove, heat being transmitted through partition 12?) which is of conductive material. Any suitable thermostat mechanism, such as that illustrated in Figs. 4, 5 and 6 indicated as an entirety by the letter T is mounted on the under side of partition 12?) interposed in one of the connections to the heating element and the thermostat mechanism is regulated to break the circuit when the temperature of said thermostatic chamber reaches a predetermined point. Since the temperature of said chamber is directly proportional to the greater temperature of the heating chamber my thermostat prevents the temperature of the heater from exceeding a desired point.

To co-operate with the eliminating heater above described I provide a wax eliminating boiler comprising a cylindrical receptacle 14 having a closed lower end and carrying intermediately of its end an inwardly projecting annular flange or seat 146. The distance from seat 141) to the top of boiler 14 is less than the height of one of the investment rings R and this seat may carry a compressible washer 15 against which the closed end of .the mold within one of the investment rings is adapted to be sealed. The upper end of boiler 14 is externally threaded and a clamping cap 16 has internally threaded engagement therewith, said cap being opened at its upper end but carrying an inwardly projecting annular flange adapted tocontact the upper edge of a crucible ring to clamp the ring and the mold therein between the seat 14?) and itself. Boiler 14 is of suitable diameter to nicely fit within the cup-like oven 13 of the heater with its lower end spaced slightly above the closed end of said oven. The boiler is supported in said position from the top of the heater by means of an annular trough 140 rigidly connected with the upper portion of said boiler and surrounding the same for the purpose of collecting overflow water and other material which as will be later shown, flows from the upper end of the mold in the step of eliminating wax from the mold.

In utilizing the above described eliminating apparatus, I prefer to adjust the thermostat to maintain the oven of my heater at a temperature adequate to bring the liquid disposed in boiler 14 to the boiling point but sufficiently low to give me a comparatively wide limit of time to carry out the next step of my process, namely the elimination of moisture from the mold. As illustrated, the lower portion of boiler 14 is filled with water to a level slightly above the seat 14?). The prepared mold with the wax model therein is clamped between washer 15 and the clamping cap-16, the lower edges of the investment ring utilized being sealed. The confined water is heated and as soon as small amounts of steam are generated the hot water is forced through the mold thereby softening and washing out the wax through the sprue pas sage in the top of the mold. Some ofthis water will, of course, overflow but will be collected by the trough 140 and thus prevented from getting into the interior of the heater or of the oven. The wax elimination step is continued beyond the point where clear Water only is forced through the sprue passage and the boiler is then lifted from the heater and the base thereof chilled under cold water. I have found that with my apparatus the wax elimination step may be successfully carried out in about live minutes, although this, of course, will vary according to the temperature at which the heater is maintained.

I next remove the mold from the eliminating boiler and place the same within the oven 3. The mold will always contain substantially the same amount of 'water or moisture since it has been saturated in my wax Cl1 n11- nation step. Consequently, the time required for evaporating the moisture in the mold will be substantially constant varying slightly with different sizes and shapes of wax models. The detached mold is permitted t0 stand in the oven of the heater until practically all the moisture has been removed. This has required from sixteen to twenty one minutes usually, although this time will vary widely according to the temperature at which the heater is maintained and according to the size and shape of the model. The point is that the time limit is comparatively long and no flame or concentrated heat is thrown directly against the investment ring or the mold. For these reasons there is no chance if this step is properly carried out of damaging the plaster of the investment material. It will also be apparent that in the heretofore dangerous step of eliminating the wax I do not subject my mold to any direct application of heat, the mold being disposed above the heater but heat only the lower portion of the boiler 14 containing the water.

After practically all the moisture is re- .moved from the mold the investment ring and mold are removed from the heater and placed in my Wfl X expander to complete the evaporation of the small residue of moisture. From three to five minutes will usually sullice and the mold may then be taken from the wax expander and should be cast shortly thereafter.

From the foregoing description it will be apparent that I expand my wax model at the time the outer investment is being applied and all during the time the investment material is setting or hardening to form. the mold. This is done without sacrificing the strength and rigidity of the inner investment constituting the actual casting surface. The expansion of the wax model as etlected by my method compensates to a large extent for the subsequent shrinkage of gold.

It will also be apparent from the foregoing description that with my method extremely hard and rigid investment material may be used which will adequately resist compres sion from the congealing gold, and to afford relatively hard and smooth casting surfaces. Due to the novel eliminating steps of my method, such investment material may be utilized for the inner investment Without I crowns and inlays which consists in forming danger of disintegratin or scorching the same, since there is no direct application of concentrated heat, such as flame to the mold itself and moreover the elimination of wax is efiected by a washing process, the mold being saturated with water rather than by an evaporation or carbonization method.

The utilization of said hard and rigid investment material containing high plaster.

content enables me to further compensate for the shrinkage of gold for the following reasons:

There is of necessit some point in the congealing of gold at which the gold becomes solid enough to overcome the resistance of investment material, and the harder the investment material is, the nearer the solid state the gold must reach before it is able to overcome the resistance of the investment and shrink in spite of it. Hence, the less will be the shrinkage. I am referring to shrinkage between two binding points on the crown or inlay. With the use of my hard investment material compensation to some extent is thus made for the shrinkage of gold, since With my high plaster content investment, the gold must congeal to a greater extent before the investment can be compressed. This compensation, with the compensation aflorded by expansion of my wax model may completely compensate for the entire shrinkage of old.

b When my method is carried out as described herein it will be possible to produce crowns and inlays of such perfection that every one may be removed from the mold and slipped to place on the tooth or on a porcelain or steel model with ordinary finger pressure and I have continuously done this for some time.

It will, of course, be understood that Various changes may be made in the form, details, arrangement and proportions of the parts without departing from the scope of the invention.

Vvhat is claimed is:

1. The method of making dental molds for crowns and inlays which consists in forming a model of wax-like substance, investing said model with plastic self-hardening material heated to a predetermined temperature and subjecting the invested model to heat to maintain the model in expanded state as said investments set.

2. The method of making dental molds for a model of wax-like substance, investing said model with a relatively thin layer of dense self-hardening material, immediately investing said model with a relatively thick layer of comparatively soft self-hardening material previously heated toa predetermined temperature and subjecting .the completely invested model to heat at a predetermined temperature whereby the model will be maintained in expanded state before and while the thin mixture of investment material, previously heated to a predetermined temperature, and subjecting the completely invested model to' heat at a predetermined temperature whereby the model will be maintained in expanded state before and while said investments set.

4; The method of making dental molds for crowns and inlays which consists in forming a model of wax-like substance, heating a relatively thin mixture of investment material at a predetermined temperature, mounting said model on a crucible former, applying a relatively dense mixture of investment material in coolstate to said model, surrounding said model with a crucible ring, pouring said heated investment mixture into said crucible ring about said model, subjecting the completely invested model and its crucible ring to heat at a predetermined temperature, whereby the model will be maintained in expanded state before and while the investments set.

5. The method of making dental molds for crowns and inlays which consists in forming a model of wax-like substance, heating a relatively thin mixture of investment material and an investment container or ring to a predetermined temperature, applying an inner relatively dense investment to said model and filling recesses in said model in so doing, placing said model within said heated investment receptacle, pouring said heated investment material into said receptacle about said model and subjecting said completely invested model and receptacle to heat at a predetermined temperature, whereby the model will be maintained in expanded state before and while the investments set.

6. The method of making dental ciOWIlS and inlays which consists in forming a model of wax-like substance, investing said model with a relatively dense mixture of water and investment material containing between forty-five and ninety-nine per cent of plaster of Paris, and a substantial proportion of silex immediately thereafter investing said model with an outer relatively thick layer of warm investment material and subjecting the conzlpletely invested model to heat at a predetermined temperature, whereby the model will be maintained in expanded state before and while the investments set and removing the wax-like substance from the mold by subjecting one end of the mold to the action of heated fluid under pressure.

7. The steps in the method of making molds for inlays and crowns which consists in covering a model of plastic material with an inner investment of self-hardening material containing in excess of forty per cent plaster of Paris and a substantial proportion of silex, permitting said investment to harden to form a mold with said model remaining therein, and removing the plastic model from the mold by subjecting one end of the mold to the action of heated fluid under pressure.

8. A process of producing dental castings which comprises forming a pattern for said casting and forming a mold around it while maintaining said pattern in an expanded condition.

9. The process of producing dental castings which comprises forming a pattern of said casting placing said pattern in a suitable fiask, expanding said pattern to a suitable size to produce the desired casting and,

forming a suitable mold around said expanded pattern.

10. The process of producing dental castings which comprises forming a pattern of the casting, placing said pattern in a suitable flask and forming a mold thereabout while maintaining the temperature of said mold material at such a point as will cause said pattern to expand beyond the approximate dimensions of the space for which said casting is intended so as to provide for the shrinkage of the casting metal, removing the pattern and then casting the metal in said mold.

11. The process of producing dental castings which includes expanding the pattern to compensate for the shrinkage of the casting metal, and forming a mold about said expanded pattern.

In testimony whereof I aflix my signature.

FREDERICK S. MEYER. 

